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bepos xray.xspatial.detect


NAME · USAGE · DESCRIPTION · PARAMETERS · EXAMPLES · BUGS · SEE_ALSO

NAME

bepos -- determine the source position from a rough position list

USAGE

bepos qpoe rough_fname bepos_tab bkden

DESCRIPTION

BEPOS determines the maximum liklihood position for each peak identified by LPEAKS, using the original qpoe file. Signal-to-noise ratios in detect cells centered on the final positions are computed and positions with snr's greater than the user-defined threshold are identified as sources.

The algorithm uses the liklihood ratio parameter technique (cf. preprint by w. cash, and memos by s. murray, ssm-102-78 ans ssm-103-78). The method was used for the Einstein IPC and ROSAT HRI and given in detail in SAO Special Report 393 by Harnden, Fabricant, Harris, and Schwarz.

previous task to run: LPEAKS, BKDEN

PARAMETERS

qpoe = "" prompt = Input qpoe file

Name of the input qpoe file. If ROSAT PSPC or Eintein IPC, PI bands must be respecified with the input qpoe filename. (ex. rp110590.qp[pi=52:201], if CELLMAP was run on the hard band)

	ROSAT PSPC:	soft band =  [pi=11:41]
			hard band =  [pi=52:201]
			broad band = [pi=11:235]

	Einstein IPC:   soft band =  [pi=2:4]
                	hard band =  [pi=5:10]
                	broad band = [pi=2:10]

rough_fname = "" prompt = input rough pos table

Required input filename. The file may be specified by supplying the rootname and a "_ruf.tab" will be appended. Each row in the table represents 1 peak position as determined by LPEAKS. BEPOS reads the following table columns of the input file:

	x ------- x pixel position in physical coordinates      
	y ------- y pixel position in physical coordinates
	cellx --- x detect cell size in arc-seconds
	celly --- y detect cell size in arc-seconds
bepos_tab = "" prompt = output bepos table file

Required output filename. The file is written in table format. The file may be specified by supplying the rootname and a "_pos.tab" will be appended. Each row in the table represent 1 source. The column titles and definition appear in the table header and are as follows:


	ra -------- right ascension in degrees
	dec ------- declination in degrees 
	x --------- x pixel position in physical coordinates      
	y --------- y pixel position in physical coordinates
        srccnts --- source strength (source counts only) in assumed point 
                    source. eq:
                    alpha = integral of the PSF over the detect cell
		    beta = integral of the PSF over the detect cell +
                           background cell
		    srcnts = ((16/9)cellcnts - framecnts)/
                              ((25/9)alpha - beta)
        bkcnts ---- background counts in the detect cell. eq:
                      bkcnts = (alpha(cellcnts+framecnts) - Beta(cellcnts)) /
			       ((25/9)Alpha - Beta)
	cellcnts -- counts at final position in a detect cell size box
	framecnts - bkdensity normalized to a detect cell size box 
	pconf ----- position error in pixels at 90% confidence
	snr ------- signal-to-noise ratio 
                    C = cellcnts
                    T = cellcnts + bkcnts
                    SNR = ((25/9)*C - T ) / (sqrt(16/9**2-1)*C + T)
	cellx ----- x detect cell size in arc-seconds
	celly ----- y detect cell size in arc-seconds
	fsflag ---- false source probability flag:
		    1 means observed photon distribution is consistent
		      with gaussian PRF for a point source
		    0 means observed photon distribution is inconsistent
		      with gaussian PRF for a point source
bkden = "" prompt = Intensity Levels

mean background density in counts/square arcminute. (computed by task BKDEN and written to this parameter as the default value by BKDEN.)

(prf_sigma = 0.0) [real]

PRF Sigma of the field in arcseconds. When 0.0, the PRF sigma is computed by keying on Telescope, instrument (from input header) & bepos.energy in a PRF lookup table (bepos.prf_table) and using the coefficients that are returned to compute the PRF.

(energy = 0.0) [real]

Energy (KeV), key look up in PRF coefficients table, coeffs are used to compute the Point Response Function Sigma.

(prf_table = "xspatialdata$prfcoeffs.tab") [string]

PRF coefficients table. Entries for ROSAT PSPC/HRI & Einstein IPC/HRI. Lookup based on value of bepos.energy. Returns coefficients assiciated with a default lookup when energy = 0.0; returns coefficients closest to the energy when exact match is not found.

(snr_thresh = 3.0) [real]

Minimum SNR threshold. Sources with computed SNR's below this threshold are rejected.

(display = 2) [int]

Display info to screen. Integer of range 0-5. The higher the number more verbose the display.

(clobber = no) [boolean]

Boolean flag specifying whether or not the output Table file can be overwritten, if it already exists.

(max_iter = 50) [int]

Maximum iterations for convergence to best position.

(max_conf_iterations = 20) [int]

Maximum iterations for confidence error calculations.

(num_conf_lev = 2) [int]

Number of confidence levels that confidence errors are determined for below.

(conf_= 2.3) [real]

The 68 percent confidence level for minimum c-statistic calculation.

(conf_= 4.61) [real]

The 90 percent confidence level for minimum c-statistic calculation.

(refine_limit = 45. ) [real]

Pixel limit between LPEAKS and BEPOS position. When the difference between the 2 positions is greater that this limit the source is thrown out.

(optimum_radius= 1.8) [real]

Optimum radius sigma. Once the value of sigma has been determined, the maximum likelihood position is determined from events in a circular region of radius ROPT*sigma, where ROPT is this parameter (OPT_RADIUS_SIGMA). This region is calculated once for each input position to this task.

(c_conv_epsilon = 1.0) [real]

Convergence minimum for c-statistic

(s_conv_epsilon = 1.0) [real]

Convergence minimum for source

(cenx_keywd = "CRPIX1" ) [string]

X center header keyword for off-axis angle calculation

(ceny_keywd = "CRPIX2" ) [string]

Y center header keyword for off-axis angle calculation

EXAMPLES

1.

de> bepos 

*** PI bands must be Respecified with input QPOE ***

Input qpoe file: xdata$rh110267.qp
Field background density in cts/arcmin**2 (0.001): 12.8558464
Input rough pos table: harlac24_ruf.tab
Output bepos table file: .

Input Qpoe: xdata$rh110267.qp
bkden = 12.8558464 (cts/sq arc-min), 0.0008928 (cts/sq pixel)


2 Rough Position(s) Input

 src#      ra            dec       physx    physy  cellcnts framecnts snr
---------------------------------------------------------------------------
   1    22:08:38.4    45:31:24.2   2380.1   2829.1      95      67     5.3
   2    22:08:41.4    45:44:33.4   2323.5   4407.7    1506     394    31.8

Writing 2 Sources to Output file:  ./harlac24_pos.tab

BUGS

SEE ALSO

Documentation on running ldetect (a detect task which runs the 5 detect tasks in sequence). (help ldetect )

Documentation on the detect algorithms and philosophy along with limitations in (help explain_detect ).

Documentation giving an overview of the detect package and run order of tasks. (help using_detect )


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